1. Field of the Invention
The invention relates to monitoring vehicular traffic over large portions of a metropolitan region, using microwave radar to observe multiple sections of roadways from elevated locations.
2. Description of the Prior Art
With the increasing density of vehicular traffic, there has been a widely-recognized need for improved traffic monitoring in metropolitan regions, which include commuter roadways and other civilian land traffic routes. Such monitoring permits real-time redirection of traffic, rapid detection of accidents and other incidents, and assembly of databases for planning future roadway improvements. Among the benefits to be derived from better traffic monitoring may be found improved safety, decreased travel time, greater fuel conservation, improved air quality, better regional planning, and overall general convenience to the public.
Various approaches for traffic monitoring, employing diverse sensor technologies, have been tried in the past or are under consideration. Most of these approaches are highly localized and employ sensors with limited spatial coverage. A recent comprehensive survey of these approaches, referred to in the survey as "wide-area" approaches, is presented in a report by Algood, et al, Oak Ridge National Laboratory Report ORNL/TM-12827, November 1994, pp. 31-63. The approaches examined in the latter report include optical, infrared, microwave, and acoustic sensors--as well as electromagnetic detectors embedded in the roadway pavement--which convey traffic data to one or more traffic control centers. These approaches, while constituting an important step forward, generally have detection ranges limited to tens or, at best, hundreds of meters. As a result, broad regional coverage using such systems would require a very large number of sensors, with associated installation and maintenance costs, as well as costs related to connecting powerlines and data links to the individual sensors. In addition, these sensors are subject to vandalism and, in the case of optical or infrared sensors, to performance degradation under conditions of poor visibility. In the case of roadway-embedded detectors, there is the further problem of traffic disruptions during their installation and repair.
Particular attention is called to several methods which use a microwave radar beam generated by a roadside sensor, as disclosed in U.S. Pat. No. 3,582,620 to Noetinger; U.S. Pat. No. 3,626,413 to Zachmann; U.S. Pat. No. 4,866,438 to Knisch; U.S. Pat. No. 4,985,705 to Stammler; and U.S. Pat. No. 5,337,082 to Fredericks. Each of the latter entails operation at short range and surveillance of a small section of roadway. The beams are apparently fixed over periods of operation.
One article (Andrews, et al, Proceedings of the IVHS America Annual Meeting, 1994, pp. 894-901) describes microwave radar apparatus used experimentally for observing road traffic over an area appreciably larger than that for the sensors cited above. The apparatus, which was situated at Logan Airport in Boston, employed a commercially-available marine radar with a conventional rotating antenna and a pulsed magnetron transmitter tube. Individual vehicles were clearly discernible and tracked on nearby airport access roads despite the presence of interfering ground echoes, the latter being generally referred to as ground "clutter". These results were achieved by comparing the composite traffic and clutter echoes to a clutter map obtained by the same apparatus in the absence of traffic. Speed was measured by tracking individual vehicles over successive antenna scans (1.7 seconds apart). However, this method of determining traffic speed may encounter difficulties in congested traffic situations (i.e., when them are multiple unresolved vehicles at the same range) or when there are "crossing targets" (i.e., vehicles which switch their relative positions between scans). Doppler techniques for speed determination are therefore preferable. But the type of radar employed in that experiment, with its magnetron transmitter, was operated in a non-coherent mode; that is, it did not preserve the microwave carrier phase-angle relationships over successive pulse transmissions. Therefore, it was unable to use Doppler techniques, which require coherent transmissions and coherent processing of the received echoes, and which are generally preferable for suppressing ground clutter. Doppler techniques have been, and continue to be, widely employed by radar designers for other radar applications.
Although not specifically intended for civilian traffic monitoring over metropolitan regions, other types of ground surveillance radar systems or apparatus are known in the art which employ certain techniques useful for the preferred embodiments of the present invention. These other types of radar systems include radars for airport surface detection equipment (ASDE) and for military airborne reconnaissance. U.S. Pat. No. 5,334,982 to Owen discloses a system for identifying aircraft and other vehicles on the surface of an airport. This system includes a radar which utilizes both a beacon interrogation return (that is, a return signal generated by an active transponder aboard a vehicle) and a conventional "skin" echo (that is, the radar echo resulting from reflection of the radar energy). An example of a skin-echo radar for airport surface surveillance is the ASDE-3 radar system cited in the latter patent; this radar system operates in the Ku-band portion of the electromagnetic spectrum and utilizes well-established radar techniques, such as: a rotating antenna, a phase-coherent transmitter employing a traveling-wave tube, advanced coherent signal processing algorithms for clutter rejection, a high-resolution waveform, and circularly-polarized transmissions for the suppression of clutter echoes caused by rainfall (rain clutter). (The polarization of an electromagnetic field describes the orientation of the electric field vector as a function of time.)
With respect to related-art airborne reconnaissance radar, this type can be broadly divided into two categories, namely radar for manned aircraft and radar for pilotless aircraft, the latter expression meaning that there are no human operators aboard. A well-known example of the first category is the JSTARS system (see, for example, Haystead, Defense Electronics, July, 1990, pp. 31-40) used for surveillance of armored surface units and other military targets. This system contains a large side-looking X-band radar installed aboard a manned aircraft of the Boeing-707 class. The radar includes a phase-coherent transmitter employing traveling-wave tubes, and a phased-array antenna which is electronically-steered in the azimuth angle dimension and mechanically rotated in the elevation angle dimension. (A phased-array antenna permits rapid, quasi-instantaneous electronic beam steering, rather than mechanical beam steering, in at least one dimension.) The JSTARS radar can operate in either a synthetic aperture radar (SAR) mode, or a Doppler-based moving target indication (MTI) mode which helps to reject the stationary ground clutter. (These modes include methods to compensate for the aircraft motion.) The radar also includes high-resolution transmitted waveforms useful for target isolation and identification. In the JSTARS system, the radar transmissions are linearly polarized and the echoes are received at the same polarization.
As for pilotless aircraft, often termed "unmanned air vehicles" (UAVs), such aircraft are gaining increasing attention as a safe, efficient, and relatively inexpensive approach for conducting surface surveillance using various on-board sensor systems, including microwave radar. A recently-initiated program, known as "Tier-2-Plus", specifies, for future implementation, an on-board radar with both SAR and MTI capabilities without specifying any particular embodiment or choice of radar components (See, for example, Entzminger, Address to the Association of Unmanned Vehicle Systems, Mar. 24, 1994.). A similar, somewhat earlier pilotless system, termed "Tier-2" (Fulghum, Aviation Week & Space Technology, Nov. 28, 1994, pg. 62), includes an on-board side-looking radar. The suppression of interfering ground clutter is more difficult in side-looking airborne radar, in contrast with radar viewing the surface along the aircraft's ground track. Finally, although not specified for surface surveillance, U.S. Pat. No. 5,097,267 to Raviv teaches a radar system aboard a pilotless aircraft, wherein the radar is characterized as an "airborne early warning" radar, which is a term customarily applied to an airborne radar for detecting hostile aircraft. The latter patent comprises a fuselage-mounted, side-looking radar with a phased-array antenna.
In view of the foregoing discussion, it is clear that there exists a long-standing need for an improved approach for more effective traffic monitoring.